During the life cycle of an oil or gas well, it is desirable to monitor and maintain well integrity. In particular, maintaining the integrity of well barriers, such as production tubing, well casing, and surrounding cement sheaths (if any), is important to ensure safe operation of the well and avoid blow-out incidents or leakage of hydrocarbons to the environment. To obtain a better understanding of the state of a well and make appropriate decisions on repairing or controlling a damaged well, it is desirable to detect the presence, extent, and location (depth) of fluid flowing near or into a wellbore since such information can be utilized in completing wells, performing remedial work, and/or determining one or more characteristics of hydrocarbon reservoirs or the surrounding formation. Monitoring downhole flows around wellbores, such as injected water, can further be of interest in reservoir characterization.
Acoustic sensors carried by downhole tools have traditionally been used to detect fluid flow into wellbores. Fluid entering the wellbore through small areas or flowing behind the wellbore wall creates turbulence and noise, which may be perceived by the acoustic sensors.